The hydrotreatment of Athabasca vacuum tower bottoms (VTB) has been examined in supercritical fluids of lower boiling-point hydrocarbon solvents (pentane, heptane, and toluene) and 10 MPa H-2 with and without activated carbon-supported Ni-Mo catalysts (in a reduced or sulfided form). For conversion of asphaltenes (AS) to maltenes (MA), the reduced metal catalysts were more active than the sulfided ones and the acid-washed activated charcoal without any metal loading was found to be the most effective catalyst for converting AS to MA. Sulfided metal catalysts, supported on either gamma-Al2O3 or activated carbon (AC), showed much higher activity for both the sulfur and nitrogen conversions than those without sulfidation. Because of its best activities for HDS/HDN, the sulfided 3% Ni-10% Mo/AC (S Ni-Mo/AC) was selected as the catalyst for further detailed studies. With respect to both AS conversion and S/N removal efficiencies for hydrotreatment of the VTB, the following priority sequence for the three supercritical solvents was obtained: toluene > heptane > pentane, following the same decreasing trend of their molecular weights, and hence, supercritical toluene was selected as the reaction medium for more detailed studies discussed in this work. The optimal temperature for hydroconversion of the VTB in supercritical toluene appears to be at around 380 degrees C, where the greatest yields of MA and the lowest yields of AS and TI were obtained, it-respective as to whether the catalyst was present or not. The catalyst was also most effective for sulfur conversion at 380 degrees C, attaining a HDS activity of about 70% compared to only about 35% in the treatment without the catalyst at the same temperature. The optimal reaction time for hydroconversion of VTB in supercrifical toluene appears to be shorter than 30-60 min, where a greater yield of MA and lower yields of AS and TI as well as less aromatic liquid products might be expected, irrespective as to whether the catalyst was present or not. The optimal time for HDS of VTB in supercritical toluene with the catalyst appears to be at 60 min. On the other hand, the effects of temperature, reaction time, and the catalyst on nitrogen conversion were less significant, compared to those on sulfur conversion, while the catalyst did show some HDN activities in the hydrotreatment of the VTB.